Shaft Member For Hydrodynamic Bearing Apparatuses And A Method For Producing The Same

ABSTRACT

A shaft member for hydrodynamic bearing apparatuses having higher dimensional accuracy produced at low costs and a method for producing the same are provided. Moreover, a shaft member for hydrodynamic bearing apparatuses having hydrodynamic grooves processed with high accuracy and a method for producing the same are provided without a large increase in the processing costs.  
     A shaft material  10  integrally having a shaft portion  11  and a flange portion  12  is formed by a forging process, and the cylindricity of a part or the entire outer circumferential surface  11   a  of the shaft portion  11  is corrected. The end face  11   b  of the shaft portion of the shaft material  10  and the end face  12   b  of the flange portion  12  on the opposite side of the shaft portion are ground relative to the corrected face  13  mentioned above, and the outer circumferential surface  10   b  of the shaft material  10  is ground relative to these end faces  11   b   , 12   b . This renders the cylindricity of the radial bearing faces  23   a   , 23   b  formed on the outer periphery of the shaft portion  21  of the produced shaft member  2  to be 3 μm or lower. Moreover, in a common forging step, a shaft material  110  integrally having the shaft portion  111  and flange portion  112  is formed, while simultaneously thrust hydrodynamic groove regions  112   a   , 112   b  are formed on both end faces of the flange portion  112.  After the forging process, in a common rolling step, radial hydrodynamic groove regions  113   a   , 113   b  are formed on the outer circumferential surface  111   a  of the shaft portion  111.  In a grinding step following the rolling process, the radial hydrodynamic groove regions  113   a   , 113   b  and the thrust hydrodynamic groove regions  112   a   , 112   b  are ground.

BACKGROUND OF THE INVENTION

The present invention relates to a shaft member for hydrodynamic bearingapparatuses which relatively rotatably supports the shaft member in theradial direction in a non-contact manner by the hydrodynamic effectwhich occurs in a radial bearing gap, and a method for producing thesame.

A hydrodynamic bearing rotatably supports a shaft member by thehydrodynamic effect of lubricating oil which occurs in a bearing gap ina non-contact manner. For example, it is used in the spindle motor ofdisk-shaped recording medium drive units such as HDDs incorporatedtherein. Hydrodynamic bearing apparatuses of this type are provided witha radial bearing portion which rotatably supports a shaft member in theradial direction in a non-contact manner, and a thrust bearing portionwhich rotatably supports the shaft member in the thrust direction in anon-contact manner. Grooves for producing a hydrodynamic pressure(hydrodynamic grooves) are formed on the inner surface of a bearingsleeve or the outer surface of the shaft member, which constitutes theradial bearing portion. Moreover, hydrodynamic grooves are formed onboth end faces of a flange portion of a shaft member which constitutes athrust bearing portion, or on the face facing it (an end face of thebearing sleeve or an end face of a thrust member fixed on the housing,or the inner bottom face of the bottom of the housing, etc.) (forexample, refer to patent document 1: Japanese Unexamined PatentPublication No. 2002-61641).

Moreover, the above hydrodynamic grooves are formed, for example, on theouter surface of the shaft member in a herringbone arrangement or aspiral arrangement. Known examples of methods for forming thehydrodynamic grooves of this type include cutting (for example, refer topatent document 2: Japanese Unexamined Patent Publication No.H08-196056), etching (for example, refer to patent document 3: JapaneseUnexamined Patent Publication No. H06-158357), among others.

BRIEF SUMMARY OF THE INVENTION

Recently, in order to deal with an increase in the information recordingdensity and rotation speed of information appliances, there is a demandfor higher rotational accuracy of spindle motors for the aboveinformation appliances. To meet this demand, higher rotational accuracyis required for hydrodynamic bearing apparatuses incorporated into theabove spindle motor.

By the way, to increase the rotational accuracy of hydrodynamic bearingapparatuses, it will be important to highly accurately control theaccuracy of a radial bearing gap and thrust bearing gap, in whichhydrodynamic pressure occurs. To control this gap appropriately, highdimensional accuracy is required for the shaft member of thehydrodynamic bearing apparatus relating to the formation of the bearinggaps mentioned above. In contrast, further increase in accuracy byconventional processing methods is difficult since they suffersignificantly high processing costs. Therefore, the presentation of anew processing method of a shaft member is desired, which has bothsatisfactory processing accuracy and processing costs.

When hydrodynamic grooves are formed on the shaft member side (forexample, on the outer surface of the shaft portion or both end faces ofthe flange portion), highly accurate processing of the hydrodynamicgrooves is required since the processing accuracy of the hydrodynamicgrooves affects the accuracy of the bearing gaps. However, to improvethe processing accuracy of the hydrodynamic grooves by employingconventional processing methods (for example, etching, cutting, etc.),the processing costs significantly increase.

A first object of the present invention is to provide a shaft member forhydrodynamic bearing apparatuses having higher dimensional accuracy atlow costs and a method for producing the same.

A second object of the present invention is to provide a shaft memberfor hydrodynamic bearing apparatuses having hydrodynamic groovesprocessed with high accuracy without a large increase in the processingcosts and a method for producing the same.

To achieve the first object, the present invention provides a shaftmember for hydrodynamic bearing apparatuses which comprises a shaftportion and a flange portion both formed by forging, and a radialbearing face facing a radial bearing gap and formed on the outerperiphery of the shaft portion, and the radial bearing face having acylindricity of 3 μm or lower. Herein, the cylindricity is defined asfollows: when a cylindrical face (the target face of the cylindricity.Herein, it refers to the radial bearing face of the shaft portion) isplaced between two geometrically correct coaxial cylindrical faces, thecylindricity is represented by the difference between the radii of thetwo coaxial cylindrical faces in the case where the interval between thetwo coaxial cylindrical faces (inscribed cylindrical face andcircumscribed cylindrical face) is rendered minimum. The radial bearingface can be any face facing the radial bearing gap which produceshydrodynamic effect, regardless of whether it has hydrodynamic groovesfor producing hydrodynamic effect.

The cylindricity of the radial bearing face formed on the outerperiphery of the shaft portion considerably affects the accuracy ofparticularly the radial bearing gap formed between the outer peripheryof the shaft portion and the bearing component (bearing sleeve, housing,etc.) facing the outer periphery of the shaft portion. That is, if thevalue of the cylindricity becomes higher, the above radial bearing gapwill not be constant in the circumferential direction or axialdirection, making the difference between the widely gapped portions andnarrowly gapped portions obvious. Accordingly, the rotational torque ofthe shaft member at the narrowly gapped bearing portions becomes higherthan at other portions, which leads to increased bearing loss, while thestiffness of the bearing becomes lower at the above widely gappedbearing portions than at other portions, which leads greater runout ofthe shaft. Moreover, if the gap is not constant in the axial direction,an undesired flow of a lubricating fluid in the axial direction mayoccur and the appropriate circulation of the lubricating fluid may beadversely affected. From these perspectives, in the present invention,the cylindricity of the radial bearing face is defined to be 3 μm orlower. Accordingly, dimensional variation of the radial bearing gap inthe circumferential direction or axial direction is suppressed, therebysuppressing the above bearing loss. This can also ensure the highstiffness of the bearing mentioned above. Therefore, the radial bearinggap between this shaft member and the bearing component facing the shaftmember can be controlled with high accuracy to realize the highrotational accuracy of a bearing apparatus comprising the shaft memberand bearing component.

In this shaft member, the perpendicularity of both end faces of theflange portion and the perpendicularity of an end face of the shaftportion, relative to the radial bearing face formed on the outerperiphery of the shaft portion, are preferably 5 μm or lower,respectively. Herein, the term “perpendicularity” is defined as follows:in the combination of a predetermined plane and a reference plane whichshould be perpendicular to each other, the perpendicularity isrepresented by the maximum value of the difference between thepredetermined plane (an end face of the flange portion or an end face ofthe shaft portion herein) and a geometric plane which is geometricallyperpendicular relative to the reference plane (the radial bearing faceherein). When the value of the perpendicularity of the end face of theflange portion is higher than 5 μm, a variation is generated in a thrustbearing gap formed between the end face and that facing it, which mayadversely affect the bearing performance including an increased bearingloss. Moreover, when the value of the perpendicularity of the end faceof the shaft portion is higher than 5 μm, it will be difficult to setthe thrust bearing gap accurately, or when the end face of the shaftportion serves as the reference plane for grinding the outer surface ofthe shaft portion and the end face of the flange portion, the processingaccuracy of these grinding surfaces may be lowered.

The above shaft member is formed of the shaft portion and flange portionrespectively by forging. Using both end faces of the shaft member (anend face of the shaft portion and an end face of the flange portionlocated on both end faces of the shaft member) as the grinding surfacesenables to perform precise grinding of the outer surface of the shaftmember using these faces as the reference planes. Accordingly, the shaftmember having the radial bearing faces whose values of cylindricity andperpendicularity are suppressed can be obtained at low costs. The shaftportion and flange portion of the above shaft member can be alsointegrally formed by forging for further cost reduction.

Forming a slanting recess portion at the corner of the shaft portion andflange portion can ensure the undercut of the grind stone in grindingboth the outer surface of the shaft portion and the end face of theflange portion. Although various methods can be usable as a method forforming this recess portion, forming by plastic processing is preferredfrom the perspective of inhibiting the production of burrs, impurities,etc., after processing.

Moreover, to achieve the first object, the present invention provides amethod for producing a shaft member for hydrodynamic bearing apparatuseswhich comprises a step of forming a shaft material having the shaftportion and flange portion integrally by forging; and a step ofcorrecting the cylindricity of a part or the entire outer surface of theshaft portion. More preferably, the present invention provides a methodfor producing a shaft member for hydrodynamic bearing apparatuses,wherein a first grinding is performed on both end faces of the shaftmaterial relative to the corrected face mentioned above, and a secondgrinding is then performed on at least the outer surface of the shaftmaterial relative to the both end faces.

In the present invention, as stated above, the cylindricity of the outersurface of the shaft portion is corrected after roughly forming of theshaft member (shaft material) having the shaft portion and flangeportion integrally by forging. Therefore, highly accurate grinding(width grinding) can be performed relative to the corrected face in thefirst grinding step described later.

For the correcting process of the cylindricity mentioned above, variousplastic processing, for example, rolling with round dies, flat dies,etc., can be used, as well as drawing compound, ironing, sizing bypressing (clipping) of split-cavity molds or the like.

In the first grinding step, both end faces located at both ends of theshaft material in the axial direction, specifically an end face of theshaft portion and an end face of the flange portion are ground. At thistime, since the end faces are ground relative to the outercircumferential surface of the shaft portion which has been subjected tothe correcting process as mentioned above, these two end faces of theshaft material can be finished with highly accurate perpendicularity andflatness.

The second grinding is then performed on the outer surface of the shaftmaterial relative to these two ground end faces of the shaft material.Both end faces of the shaft material, which are the reference planes,have been highly accurately finished in the first grinding step. Hence,the target to be processed, i.e., the outer circumferential surface ofthe shaft material can also be finished highly accurately. The secondgrinding process is performed on at least a portion which will be theradial bearing face of the outer circumferential surface of the shaftmaterial. Additionally, the process can also be performed on the outercircumferential surface of the flange portion. Furthermore, it can beperformed on the other (on the shaft portion side) end face of theunground flange portion. In this second grinding step, theseto-be-ground faces can be finished at a time by using grind stones(formed grind stone) having the outline shapes corresponding to theseto-be-ground faces of the shaft material.

By following the above-mentioned procedure, the shaft member in whichthe radial bearing face has the cylindricity of 3 μm or lower and bothend faces of the flange portion and the end face of the shaft portionhave the perpendicularity of 5 μm or lower, respectively, can beproduced at low costs.

The above shaft member for hydrodynamic bearing apparatuses can beprovided as a hydrodynamic bearing apparatus which comprises a bearingsleeve into which the shaft member is inserted at its inner surface; aradial bearing portion which produces pressure by the hydrodynamiceffect which occurs in a radial bearing gap between the outer peripheryof the shaft portion and the inner periphery of the bearing sleeve tosupport the shaft portion in the radial direction in a non-contactmanner; a first thrust bearing portion which produces pressure by thehydrodynamic effect of a fluid which occurs in a thrust bearing gap onone end side of the flange portion to support the flange portion in thethrust direction in a non-contact manner; and a second thrust bearingportion which produces pressure by the hydrodynamic effect of the fluidoccurring in the thrust bearing gap on the other end side of the flangeportion to support the flange portion in the thrust direction in anon-contact manner.

In this case, for example, hydrodynamic grooves for producing thehydrodynamic effect of the fluid can be formed asymmetrically in theaxial direction on one of the outer circumferential surface of the shaftportion facing the radial bearing gap and the inner periphery face ofthe bearing sleeve opposing this outer circumferential surface.

The above hydrodynamic bearing apparatus can be provided as a motorwhich comprises a hydrodynamic bearing apparatus, a rotor magnet and astator coil.

To achieve the second object, the present invention provides a shaftmember for hydrodynamic bearing apparatuses which is a metallic shaftmember for hydrodynamic bearing apparatuses which integrally comprisesthe shaft portion and the flange portion, in which a radial hydrodynamicgroove region comprising the hydrodynamic grooves and demarcationportions demarcating each hydrodynamic groove is formed by plasticprocessing on the outer periphery of the shaft portion, and the outercircumferential surfaces of the demarcation portions in the radialhydrodynamic groove region are grinding surfaces. The demarcationportions herein refer to the portions which demarcate the hydrodynamicgrooves, including the so-called ridges between the hydrodynamicgrooves. Moreover, when the hydrodynamic grooves are formed with a slantarrangement in the axial direction, so-called smooth portions whichdivide those slanting hydrodynamic grooves in the axial direction arealso included in the demarcation portions.

In the present invention, as stated above, the radial hydrodynamicgroove region comprising the hydrodynamic grooves and demarcationportions demarcating each hydrodynamic groove is formed by plasticprocessing on the outer periphery of the shaft portion of the shaftmember. Hence, for example, cutting powders are not produced unlike incutting, thereby saving materials. Compared to processing methods byetching, the trouble of performing masking preliminarily for preventingof corrosion can be dispensed with, and processing costs can be thusgreatly reduced on the whole. Moreover, the present invention ischaracterized in that the outer circumferential surfaces of thedemarcation portions in the radial hydrodynamic groove region aregrinding surfaces. These grinding surfaces are obtained by grinding theouter diameter portions of the demarcation portions (the top portionsadjacent to the hydrodynamic grooves) demarcating the hydrodynamicgrooves of the radial hydrodynamic groove regions formed by plasticprocessing. Accordingly, precise processing of the hydrodynamic grooveregion, which cannot be achieved only by plastic processing, is enabled,and the dimensional accuracy of the outer diameter and surface roughnesscan be accurately obtained. Therefore, according to the presentinvention, improved processing accuracy and reduced processing costs canbe both achieved, such radial bearing gap in hydrodynamic bearingapparatuses can be controlled highly accurately.

Such a hydrodynamic groove region can be formed, for example, on bothend faces of the flange portion formed integrally with the shaft portionby plastic processing. In this case, the flange portion is soconstructed that thrust hydrodynamic groove regions comprising thehydrodynamic grooves and demarcation portions demarcating eachhydrodynamic groove are formed on its both end faces and the end face inaxial direction of the demarcation portions in these thrust hydrodynamicgroove regions are grinding surfaces.

The radial hydrodynamic groove region can be formed, for example, by arolling process or a forging process. Alternatively, both the radialhydrodynamic groove region and thrust hydrodynamic groove region can beformed by a forging process. Alternatively, the shaft portion and flangeportion, in which these hydrodynamic groove regions are formed,respectively, can be formed, for example, integrally by forging.

To achieve the second object, the present invention also provides amethod for producing a shaft member for hydrodynamic bearing apparatuseswhich comprises a shaft portion and a flange portion integrally, and aradial hydrodynamic groove region comprising hydrodynamic grooves anddemarcation portions demarcating each hydrodynamic groove on the outerperiphery of the shaft portion, the method comprising forming a radialhydrodynamic groove region by plastic processing on the outer peripheryof the shaft portion of the shaft material, and then grinding a portionincluding an outer diameter portion of the demarcation portion in theradial hydrodynamic groove region.

According to such a producing method, both an improvement in theprocessing accuracy of the radial hydrodynamic groove region andreduction of the processing costs can be achieved. Moreover, forming theshaft material which integrally has the shaft portion and the flangeportion by forging can realize further reduction of processing costs, orreduction of the cycle time per product.

Examples of the plastic processing of the radial hydrodynamic grooveregion employed include a forging process. In this case, both the shaftmaterial and radial hydrodynamic groove region can be formed by forging,and forging of them can be performed simultaneously. Accordingly, such aprocessing step can be simplified and the cycle time required forprocessing can be even reduced.

In the shaft portion of the shaft material, it is possible to perform arolling process for correcting the cylindricity of a portion includingthe radial hydrodynamic groove region of the shaft portion. In thiscase, for example, both the formation of the radial hydrodynamic grooveregion and the correction of the cylindricity of a portion including theradial hydrodynamic groove region of the shaft portion can be performedby rolling simultaneously so that such a processing step can besimplified and the cycle time can be shortened. Thus, the massproductivity of the product can be dramatically improved.

Alternatively, it is possible to perform forming the shaft material andforming the thrust hydrodynamic groove region comprising thehydrodynamic grooves and demarcation portions demarcating eachhydrodynamic groove on both end faces of the flange portion both byforging, and to simultaneously perform forging of both. Accordingly, theprocessing steps relating to the formation of the shaft material andthrust hydrodynamic groove region can be simplified to shorten themachining time.

The above shaft member for hydrodynamic bearing apparatuses can bepresented, for example, as a hydrodynamic bearing apparatus whichcomprises a shaft member for hydrodynamic bearing apparatuses; and asleeve member into which this shaft member is inserted at its innersurface and which forms a radial bearing gap between itself and theshaft member, which retains the shaft member and sleeve member in anon-contact manner by the hydrodynamic effect of a fluid occurring inthe radial bearing gap. The bearing sleeve can be formed, for example,from an oil-containing sintered metal, and a thrust hydrodynamic grooveregion can be formed on an end face in the axial direction of the sleeveinstead of the end face of the flange portion.

The above hydrodynamic bearing apparatus can be provided as a motorcomprising this hydrodynamic bearing apparatus, a rotor magnet and astator coil.

According to the present invention, the outer circumferential surface ofthe shaft portion and the end face of the flange portion of the shaftmember involved in the formation of the radial bearing gap and thrustbearing gap can be processed highly accurately at low costs. Therefore,these bearing gaps of the hydrodynamic bearing apparatus incorporatingthe shaft member can be controlled highly accurately. As a result, highrotational accuracy can be imparted to the above hydrodynamic bearingapparatus.

Moreover, according to the present invention, the hydrodynamic groovesformed on the shaft member can be processed accurately without anincrease in such processing costs. Moreover, the bearing performance ofthe hydrodynamic bearing apparatus integrating this shaft member can beexerted stably for a long term by controlling the bearing gap in thehydrodynamic bearing apparatus highly accurately.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a side elevational view of a shaft member for the hydrodynamicbearing apparatus according to the first embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of a spindle motor for an informationappliance integrating a hydrodynamic bearing apparatus comprising ashaft member.

FIG. 3 is a longitudinal sectional view of a hydrodynamic bearingapparatus.

FIG. 4 is a longitudinal sectional view of a bearing sleeve.

FIG. 5 is a side elevational view of a shaft material formed by aforging process.

FIG. 6 is a schematic illustration of a correcting process (rollingprocess) by round dies.

FIG. 7 is a schematic illustration of a correcting process (rollingprocess) by flat dies.

FIG. 8 is a schematic illustration showing an example a grindingapparatus according to the width grinding step of a shaft material.

FIG. 9 is a partial cross-sectional view showing an example of agrinding apparatus according to the width grinding step.

FIG. 10 is a schematic illustration showing an example of a grindingapparatus according to the full-face grinding step of a shaft material.

FIG. 11 is a schematic illustration showing an example of a grindingapparatus according to the grinding finishing step of a shaft material.

FIG. 12 is an expanded sectional view of the vicinity of the cornerbetween the shaft portion and flange portion of a shaft member.

FIG. 13 is a side elevational view of a shaft member for a hydrodynamicbearing apparatus according to the second embodiment of the presentinvention.

FIG. 14 is a top view of the flange portion of a shaft member seen fromthe direction of arrow A.

FIG. 15 is a bottom view of the flange portion of a shaft member seenfrom the direction of arrow B.

FIG. 16 is a longitudinal sectional view of a hydrodynamic bearingapparatus comprising a shaft member.

FIG. 17 is a side elevational view of a shaft material formed by aforging process.

FIG. 18 is a top view of the flange portion of a shaft material seenfrom the direction of arrow A.

FIG. 19 is a bottom view of the flange portion of a shaft material seenfrom the direction of arrow B.

FIG. 20 is an expanded sectional view of a thrust hydrodynamic grooveregion formed on the end face of a flange portion on the side oppositeto the shaft portion prior to grinding.

FIG. 21 is an expanded sectional view of a thrust hydrodynamic grooveregion after being ground.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention will be described below withreference to FIGS. 1-12.

FIG. 2 conceptionally shows a constitutional example of a spindle motorfor an information appliance incorporating a hydrodynamic bearingapparatus 1 according to the first embodiment of the present invention.This spindle motor for an information appliance is used for disk driveunits such as HDDs, and comprises the hydrodynamic bearing apparatus 1which rotatably supports a shaft member 2 in a non-contact manner, adisk hub 3 which is mounted on the shaft member 2, for example, a statorcoil 4 and a rotor magnet 5 facing each other across a gap in the radialdirection, and a bracket 6. The stator coil 4 is mounted on the outerperiphery of the bracket 6, and the rotor magnet 5 is mounted on theinner periphery of the disk hub 3. The bracket 6 has the hydrodynamicbearing apparatus 1 mounted on its inner periphery. Moreover, the diskhub 3 retains one or more disks D such as magnetic disks on its outerperiphery. In this spindle motor for an information appliance, when thestator coil 4 is energized, the rotor magnet 5 is rotated by theexcitation between the stator coil 4 and rotor magnet 5, whereby thedisk hub 3 and the disk D retained by the disk hub 3 is rotatedunitarily with the shaft member 2.

FIG. 3 shows the hydrodynamic bearing apparatus 1. This hydrodynamicbearing apparatus 1 is mainly constituted of a housing 7 having a bottom7 b at its one end, a bearing sleeve 8 fixed on the housing 7, and ashaft member 2 inserted at the inner periphery of the bearing sleeve 8.For the sake of explanation, the bottom 7 b side of the housing 7 isreferred to as the lower side, and the side opposite to the bottom 7 bis referred to as the upper side in the following description.

As shown in FIG. 3, the housing 7 is constituted of, for example, a sideportion 7 a formed of a resin material such as LCP, PPS and PEEK in theform of a cylinder, and a bottom 7 b located at one end side of the sideportion 7 a and, for example, formed of a metallic material. In thisembodiment, the bottom 7 b is formed separately from the side portion 7a, is retrofitted on the lower inner periphery of the side portion 7 a.In a part of the annular region os the upper end face 7 b 1 of thebottom 7 b, hydrodynamic grooves are formed, for example, in the form ofa spiral, as a portion for producing hydrodynamic pressure, although notshown in the Figs. In this embodiment, the bottom 7 b is formedseparately from the side portion 7 a, and is fixed on the lower innerperiphery of the side portion 7 a. It can be, however, formed integrallywith the side portion 7 a, for example, from a resin material. At thistime, the hydrodynamic grooves provided on the upper end face 7 b 1 canbe molded simultaneously with the injection molding of the housing 7comprising the side portion 7 a and bottom 7 b, which can dispense withthe trouble of forming the hydrodynamic grooves on the bottom 7 b.

The bearing sleeve 8 is formed of, for example, a porous body made of asintered metal, especially a porous body of a sintered metal comprisingcopper as a main ingredient in the form of a cylinder, and is fixed at apredetermined position on an inner surface 7 c of the housing 7.

Throughout an inner surface 8 a of the bearing sleeve 8 or in a part ofits cylindrical face region, a radial hydrodynamic pressure producingpart is formed. In this embodiment, for example, as shown in FIG. 4, aregion, in which a plurality of hydrodynamic grooves 8 a 1, 8 a 2 arearranged in a herringbone shape, is formed at two axially separatedpositions. The upper hydrodynamic groove 8 a 1 is formed asymmetricallyin the axial direction relative to the axial center m (the axial centerof the region between the upper and lower slanted grooves), the axialdimension X1 of the region above the center m in the axial direction islarger than the axial dimension X2 of the region therebelow.

Although not shown in the Figs., for example, a region in which aplurality of hydrodynamic grooves are arranged spirally is formedthroughout the lower end face 8 b of the bearing sleeve 8 or in a partof annular region, as a portion for producing thrust hydrodynamicpressure.

A sealing member 9 as a sealing means is formed of, for example, a softmetallic material such as brass and other metallic materials, or a resinmaterial in a ring shape, as shown in FIG. 3. The sealing member 9 ispress-fitted to the upper inner periphery of the side portion 7 a of thehousing 7, and is fixed by means of adhesion or the like. In thisembodiment, the inner surface 9 a of the sealing member 9 is formed inthe shape of a cylinder, and the lower end face 9 b of the sealingmember 9 is in contact with the upper end face 8C of the bearing sleeve8.

As shown in FIG. 1, the shaft member 2 is formed of a metallic materialsuch as stainless steel, and has a T-shaped cross section integrallycomprising a shaft portion 21 and a flange portion 22 provided at thelower end of the shaft portion 21. On the outer periphery of the shaftportion 21, as shown in FIG. 3, radial bearing faces 23 a, 23 b facingthe formation region of two hydrodynamic grooves 8 a 1, 8 a 2 formed onthe inner surface 8 a of the bearing sleeve 8 are formed at two axiallyseparated positions. Above one of the radial bearing faces, the face 23a and a tapered face 24 whose diameter gradually decreases toward theshaft tip are formed adjacently. Further thereabove, a cylinder face 25,which serves as a mounting portion of the disk hub 3, is formed. Annularrecess portions 26, 27, 28 are formed between the two radial bearingfaces 23 a, 23 b, between the other radial bearing face 23 b and flangeportion 22, and between the tapered face 24 and cylinder face 25,respectively.

On both end faces of the flange portion 22, thrust bearing faces 22 a,22 b facing the hydrodynamic groove regions formed on the lower end face8 b of the bearing sleeve and the upper end face 7 b 1 of the bottom 7b, respectively, are formed.

Between the tapered face 24 of the shaft portion 21 and the innersurface 9 a of the sealing member 9 facing the tapered face 24, anannular sealing space S, whose radial dimension gradually increasesupwardly from the bottom 7 b side of the housing 7, is formed. In thehydrodynamic bearing apparatus 1 after being assembled (refer to FIG.3), the oil level is within the range of the sealing space S.

In the thus constructed hydrodynamic bearing apparatus 1, when the shaftmember 2 is rotated, the pressure of a lubricating oil film formed inthe radial bearing gap between the formation regions (two positions:upper and lower) of the hydrodynamic grooves 8 a 1, 8 a 2 of the innerperiphery of the bearing sleeve 8 and the radial bearing faces 23 a, 23b of the shaft portion 21 facing these regions, respectively, isincreased by the hydrodynamic effect of the hydrodynamic grooves 8 a 1,8 a 2. A first radial bearing portion R1 and a second radial bearingportion R2 which rotatably support the shaft member 2 in the radialdirection in a non-contact manner by the pressure of these oil films arethen formed. The pressures of a first thrust bearing gap between thehydrodynamic groove region formed on the lower end face 8 b of thebearing sleeve 8 and the thrust bearing face 22 a of the upper side (theshaft portion side) of the flange portion 22 facing this hydrodynamicgroove region and a lubricating oil film formed on a second thrustbearing gap between the hydrodynamic groove region formed on the upperend face 7 b 1 of the bottom 7 b and the thrust bearing face 22 b of thelower side (opposite to the shaft portion side) of the flange portion 22facing this face are increased by the hydrodynamic effect of thehydrodynamic grooves. A first thrust bearing portion T1 and a secondthrust bearing portion T2 which rotatably support the shaft member 2 inthe thrust direction in a non-contact manner by the pressures of theseoil films are then formed.

The method for producing the shaft member 2 constituting the abovehydrodynamic bearing apparatus 1 will be described below.

The shaft member 2 is produced in mainly two steps: (A) forming step and(B) grinding step. In this embodiment, one of these steps, the (A)forming step includes a forging process (A-1) and a correcting process(A-2), and the (B) grinding step includes width grinding (B-1),full-face grinding (B-2) and finish grinding (B-3).

(A) Forming Step

(A-1) Forging Process

To begin with, a bar material made of metal such as stainless steelwhich is a material of the shaft member 2 to be formed is cold-forged toform the shaft material 10 having a T-shaped cross section andintegrally having the shaft portion 11 and flange portion 12, as shownin FIG. 5. The cold-forging method used may be any of extrusion,upsetting, heading or the like, or combinations of them. In the examplesshown in FIG. 5, the outer circumferential surface 11 a of the shaftportion 11 after being subjected to the forging process has such adifferent diameter shape that the tapered face 14 is disposedtherebetween, but may be formed to have a uniform diameter throughoutits length by dispensing with the tapered face 14.

As mentioned above, forming the shaft material 10 by forging does notproduce cutting allowance and can reduce wasted materials compared withforming the shaft material 10 having a similar shape by, for example,cutting or the like. Moreover, since it is a pressing operation, thecycle time per piece of the shaft material 10 can be improved, therebyimproving the productivity.

(A-2) Correcting Process

Subsequently, the outer circumferential surface 11 a of the shaftportion of the shaft material 10 after being subjected to the forgingprocess is subjected to a plastic processing for correcting thecylindricity. This improves the cylindricity of the face 13 subjected tothe correcting process, of the outer circumferential surface of theshaft portion 11 a of the shaft material 10 so that it falls within arequired range (for example, 10 μm or lower). At this time, thecorrecting process of the cylindricity employed may be, for example, arolling process by using round dies 34, flat dies 35, etc., as shown inFIG. 6 or FIG. 7. Various other processing methods such as drawing,ironing, sizing process by pressing (clipping) split-cavity molds, etc.,can be employed. The correcting process is conducted throughout thelength of the outer circumferential surface of the shaft portion 11, orcan be conducted on a part thereof. When only a part thereof iscorrected, its processed region includes at least the region which willbe the radial bearing faces 23 a, 23 b of the shaft member 2.

(B) Grinding Step

(B-1) Width Grinding Process

The end face 11 b of the shaft portion and the end face 12 b of theflange portion 12 on the opposite side of the shaft portion (refer toFIG. 5), which will be the end faces of the shaft material 10 subjectedto the correcting process, is ground relative to the corrected face 13mentioned above of the outer circumferential surface of the shaftportion 11 a (first grinding step). A grinding apparatus 40 used in thisgrinding step comprises, for example, a carrier 41 which retains aplurality of the shaft material 10 as workpieces, and a pair of grindstones 42, 42 which grinds the end face 11 b of the shaft portion of theshaft material 10 retained by the carrier 41 and the end face 12 b ofthe flange portion 12 on the side opposite to the shaft portion, asshown in FIG. 8.

As shown in FIG. 8, a plurality of notches 43 are provided on a part ofthe circumferential region of the outer circumferential edge of thecarrier 41 at an equal pitch in the circumferential direction. The shaftmaterial 10 is contained in the notch 43 with its correcting processface 13 in angular contact with the inner face 43 a of the notch 43. Thecorrecting process face 13 of the shaft material 10 protrudes slightlyfrom the outer circumferential surface of the carrier 41, and on theouter diameter side of the carrier, a belt 44 is provided in a tensionedstate to bind the protruding portions of the shaft material 10 from theouter diameter side. On both end sides of the carrier 41 of the shaftmaterial 10 contained in the notch 43 in the axial direction, a pair ofgrind stones 42, 42 are coaxially disposed with their end faces(grinding surfaces) facing each other at a predetermined interval.

As the carrier 41 rotates, the shaft material 10 is sequentially loadedinto the notch 43 from a determined position. The loaded material 10traverses the end faces of the rotating grind stones 42, 42 from theirouter diameter edge toward the inside diameter edge, while beingprevented from falling off from the notch 43 by binding of the belt 44.Accordingly, both end faces of the shaft material 10, i.e., the end face11 b of the shaft portion and the end face 12 b of the flange portion 12on the side opposite to the shaft portion are ground by the end faces ofthe grind stones 42, 42. At this time, since the corrected face 13 ofthe shaft material 10 is supported by the carrier 41 and this correctedface 13 has high cylindricity. Therefore, if the perpendicularity of therotation axis of the grind stone 42 and the grinding surface of thegrind stone 42 and the parallelism of the rotation axis of the grindstone 42 and the rotation axis of the carrier 41, etc., are controlledin advance with highly accuracy, relative to this corrected face 13, theabove-mentioned both end faces 11 b, 12 b of the shaft material 10 canbe finished with high accuracy, enabling to suppress the value of theperpendicularity relative to the corrected face 13. Moreover, the widthof the shaft material 10 in the axial direction (the overall lengthincluding the flange portion 12) can be finished to have a predeterminedsize.

(B-2) Full-Face Grinding Process

Subsequently, the outer circumferential surface 10 b of the shaftmaterial 10 and the end face 12 a on the shaft portion side of theflange portion 12 are ground relative to both end faces 11 b, 12 b ofthe ground shaft material (second grinding step). The grinding apparatusused in this grinding step is, for example, plunge-ground by the grindstone 53 with the back plate 54 and pressure plate 55 pressed againstboth end faces of the shaft material 10, as shown in FIG. 10. Thecorrected face 13 of the shaft material 10 is rotatably supported by ashoe 52.

The grind stone 53 is a formed grind stone which comprises a grindingsurface 56 corresponding to the outer circumferential surface shape ofthe shaft member 2 as a finished product. The grinding surface 56comprises a cylinder grinding portion 56 a which grinds the outercircumferential surface 11 a throughout the axial length of the shaftportion 11 and the outer circumferential surface 12 c of the flangeportion 12; and a plane grinding portion 56 b which grinds the end face12 a on the shaft portion side of the flange portion 12. In the exampleshown in FIG. 10, the grind stone 53 comprises, as the cylinder grindingportion 56 a, portions 56 a 1, 56 a 2, which grind the regionscorresponding to the radial bearing faces 23 a, 23 b of the shaft member2, a portion 56 a 3, which grinds the region corresponding to thetapered face 24, a portion 56 a 4, which grinds the region correspondingto the cylinder face 25, portions 56 a 5-56 a 7, which grind the recessportions 26-28, respectively, and a portion 56 a 8, which grinds theouter circumferential surface 12 c of the flange portion 12.

Grinding in the grinding apparatus 50 of the above constitution isperformed in the following procedure. To begin with, the grind stone 53is fed in a diagonal direction (the direction of arrow 1 in FIG. 10)with the shaft material 10 and grind stone 53 rotating, and the planegrinding portion 56 b of the grind stone 53 is pressed against the endface 12 a on the shaft portion side of the flange portion of the shaftmaterial 10, to mainly grind the end face 12 a on the shaft portionside. This causes the end face 12 a on the shaft portion side in theflange portion 22 of the shaft member 2 to be ground. Subsequently, thegrind stone 53 is fed in the direction perpendicular to the rotationaxis of the shaft material 10 (the direction of arrow 2 in FIG. 10), andthe cylinder grinding portion 56 a of the grind stone 53 is pressedagainst the outer circumferential surface 11 a of the shaft portion 11of the shaft material 10 and the outer circumferential surface 12 c ofthe flange portion 12 to grind the faces 11 a, 12 c. Accordingly, out ofthe outer circumferential surface of the shaft portion 21 of the shaftmember 2, the regions 13 a, 13 b corresponding to the radial bearingfaces 23 a, 23 b of the shaft material 10, the tapered face 24 and theregion 15 corresponding to the cylinder face 25, and the outercircumferential surface 22 c of the flange portion 22 are ground, andthe recess portions 26-28 are formed. Note that in the above grinding,for example, as shown in FIG. 10, it is preferable to perform grindingwhile measuring the remaining grinding allowance by a measurement gauge57.

In this second grinding step, since the accuracy setting has beenperformed of the perpendicularity of both end faces 11 b, 12 b of theshaft material 10 beforehand in the width grinding, each of theto-be-ground surfaces can be ground highly accurately.

(B-3) Finish Grinding Process

(B-2) Among the faces which have been ground in full-face grinding, theradial bearing faces 23 a, 23 b of the shaft member 2 and regions 13 s,13 b, 15 corresponding to the cylinder face 25 are subjected to finalfinish grinding. A grinding apparatus used in this grinding, forexample, performs plunge grinding by the grind stone 63, while rotatingthe shaft material 10 held between the back plate 64 and pressure plate65 by the cylinder grinder shown in FIG. 11. The shaft material 10 isrotatably supported by a shoe 62. A grinding surface 63 a of the grindstone 63 comprises the first cylinder grinding portion 63 a 1, whichgrinds the regions 13 a, 13 b corresponding to the radial bearing faces23 a, 23 b, and the second cylinder grinding portion 63 a 2, whichgrinds the region 15 corresponding to the cylinder face 25.

In the thus constructed grinding apparatus 60, the rotating grind stone63 is provided with the feed in the radial direction so that the radialbearing faces 23 a, 23 b and the regions 13 a, 13 b, 15 corresponding tothe cylinder face 25 are ground respectively and these regions arefinished with a final surface accuracy. In this embodiment, the regionscorresponding to the radial bearing face 23 a, 23 b and the regioncorresponding to the cylinder face 25 are both subjected to finishgrinding, the grinding of the region corresponding to the cylinder face25 may be dispensed with.

After performing the (A) forming step and (B) grinding step discussedthe above, heat treatment and cleaning process, if necessary, can beperformed to complete the shaft member 2 shown in FIG. 1.

The shaft member 2, as long as it is produced by the production methodmentioned above, can be finished to have the cylindricity of the radialbearing faces 23 a, 23 b formed on the outer periphery of the shaftportion 21 of, for example, 3 μm or lower (desirably 1.5 μm or lower).This allows, for example, variation in the radial bearing gap formedbetween itself and the inner periphery of the bearing sleeve 8 of in thehydrodynamic bearing apparatus 1 in the circumferential direction oraxial direction to fall within a predetermined range, preventing bearingperformance from being adversely affected by the variation of the aboveradial bearing gap. Therefore, such a radial bearing gap can becontrolled with high accuracy, and the rotational accuracy ofhydrodynamic bearing apparatuses of this type can be maintained at ahigh level. Note that in this embodiment, not only the radial bearingface 23 a, 23 b but also the region corresponding to the cylinder face25 are subjected to finish grinding (refer to FIG. 11), the cylinderface 25 is also finished to have the above cylindricity. Therefore, themounting accuracy (perpendicularity, etc.) of mounting components suchas the disk hub 3 on the shaft member 2 is increased, contributing tothe improvement in the motor performance.

It is possible to form the shaft member 2 in which the perpendicularityof both end faces of the flange portion 22 (thrust bearing faces) 22 a,22 b and the perpendicularity of the end face 21 b of the shaft portionare both 5 μm or lower, relative to the radial bearing faces 23 a, 23 bformed on the outer periphery of the shaft portion 21 according to theabove production method. Among them, the thrust bearing faces 22 a, 22 bformed on both end faces of the flange portion 22 form the thrustbearing gap between the face opposing them (the lower end face 8 b ofthe bearing sleeve 8 and the upper end face 7 b 1 of the bottom 7 b ofthe housing 7, etc.) and themselves. Therefore, the numerical value ofsuch perpendicularity can be thus suppressed to a low level, wherebyvariation in of the above thrust bearing gap can be reduced. Moreover,the end face 21 b of the shaft portion serves not only as the referenceplane for grinding the outer circumferential surface of the shaftportion 21 and the upper end face of the flange portion 22 (thrustbearing face 22 a side), but also as the reference plane for setting theabove thrust bearing gap. Accordingly, by suppressing the numericalvalue of the perpendicularity of the end face 21 b of the shaft portionto a low level, and such a grinding face, as well as the thrust bearinggap, can be controlled highly accurately.

Note that in the above description, in the full-face grinding shown inFIG. 10, the cylinder grinding of the outer circumferential surface 10 bof the shaft material 10 and the plane grinding of the end face 12 a onthe shaft portion side of the flange portion 12 are performed by thecommon grind stone 53, but both grinding may be performed by differentgrind stones.

In the above description, the case where the recess portions 26-28 ofthe shaft member 2 are formed in the full-face grinding (B-2) shown inFIG. 10 was exemplified. However, these recess portions 26-28 may besubjected to the plastic processing (for example rolling) simultaneouslyin correcting process shown in FIGS. 6 and 7. In this case, inparticular the recess portion 27 of the corner between the shaft portion21 and flange portion 22 is formed obliquely as shown in FIG. 12. Thisallows the recess portion 27 to also serve as an undercut of the grindstone 53 for grinding the end face 12 a on the shaft portion side of theflange portion 12 and the outer circumferential surface of the shaftportion 11 a simultaneously in the full-face grinding (refer to FIG.10).

In the embodiments described above, the case where the radial bearingfaces 23 a, 23 b of the shaft member 2 and thrust bearing faces 22 a, 22b are all smooth surfaces having no hydrodynamic grooves wasexemplified, but hydrodynamic grooves may be formed on these bearingfaces. In this case, the radial hydrodynamic grooves can be formed byrolling or forging, and the thrust hydrodynamic groove can be formed bypressing or forging, at the stage preceding the full-face grinding shownin FIG. 10.

A second embodiment of the present invention will be described belowwith reference to FIGS. 13-21. Note that the parts and components havingthe same constitution and action as the constitution (first embodiment)shown in FIGS. 1-12 are denoted by the identical reference numerals, andrepeated explanations are omitted.

FIG. 16 shows a hydrodynamic bearing apparatus 101 according to thesecond embodiment of the present invention. This hydrodynamic bearingapparatus 101 is also used in a spindle motor for disk drive units shownin FIG. 2 incorporated therein, and constitutes a motor together with,for example, a disk hub 3, stator coil 4, rotor magnet 5 and bracket 6shown in the same Figs (FIG. 2). The hydrodynamic bearing apparatus 101comprises a housing 7 having a bottom 7 b at its one end, a bearingsleeve 8 fixed on to the housing 7, a shaft member 102 inserted at theinner periphery of the bearing sleeve 8, and a sealing member 9 as itsmain components. Note that also in this embodiment, for the sake ofexplanation, the side of the bottom 7 b of the housing 7 is referred toas the lower side, and the side opposite to the bottom 7 b is referredto as the upper side in the description below.

As shown in FIG. 13, the shaft member 102 is formed of, for example, ametallic material such as stainless steel, and has a T-shaped crosssection integrally comprising a shaft portion 121 and a flange portion122 provided at the lower end of the shaft portion 121. In a part of theouter periphery of the shaft portion 121, a cylinder region, radialhydrodynamic groove regions 123 a, 123 b are formed at two axiallyseparated positions. Accordingly, in this embodiment, an inner surface 8a of a bearing sleeve 8 facing the radial hydrodynamic groove regions123 a, 123 b is a cylindrical face having no hydrodynamic grooves andhaving a circular cross section.

These two upper and lower hydrodynamic groove regions 123 a, 123 bcomprise a plurality of hydrodynamic grooves 123 a 1, 123 b 1 anddemarcation portions 123 a 2, 123 b 2 demarcating the hydrodynamicgrooves 123 a 1, 123 b 1, respectively. In this embodiment, as shown inFIG. 1, they are both in a herringbone shape. Among them, the upperradial hydrodynamic groove region 123 a is formed asymmetrically in theaxial direction relative to the axial center m (the center in the axialdirection of the region between the upper and lower slanted grooves),and the axial dimension X1 of the region above the axial center m islarger than the axial dimension X2 of the region therebelow.

Throughout the upper end face of the flange portion 122 or in a part ofits annular region, for example, as shown in FIG. 14, a thrusthydrodynamic groove region 122 a is formed. Moreover, in a part of itsannular region of the lower end face of the flange portion 122, forexample, as shown in FIG. 15, a thrust hydrodynamic groove region 122 bis formed. These thrust hydrodynamic groove regions 122 a, 122 bcomprise respectively a plurality of hydrodynamic grooves 122 a 1, 122 b1 and demarcation portions 122 a 2, 122 b 2 demarcating the hydrodynamicgroove 122 a 1, 122 b 1. In this embodiment, as shown in FIGS. 14 and15, each of the region forms a spiral shape. Note that the thrusthydrodynamic groove regions 122 a, 122 b may be in the shape, forexample, of a herringbone shape or the like, without being limited tothe shape shown particularly. Alternatively, each of the upper and lowerfaces may have different hydrodynamic groove shapes.

Above one of the hydrodynamic groove regions, the radial hydrodynamicgroove region 123 a, a tapered face 124, of which diameter graduallydecreases toward the shaft tip, is formed adjacently, and a cylinderface 125, which will be a mounting portion of the disk hub 3, is formedfurther thereabove. Annular recess portions 126, 127, 128, are formedbetween the two radial hydrodynamic groove regions 123 a, 123 b, betweenthe other radial hydrodynamic groove region 123 b and the flange portion122, and between the tapered face 124 and the cylinder face 125,respectively.

Between the tapered face 124 of the shaft portion 121 and the innersurface 9 a of a sealing member 9 facing the tapered face 124, anannular sealing space S, whose size in the radial direction is graduallyincreased upwardly from the bottom 7 b side of the housing 7 is formed.In the hydrodynamic bearing apparatus 1 after being assembled (refer toFIG. 16), the oil level is maintained within the range of the sealingspace S.

In the thus constructed hydrodynamic bearing apparatus 101, when theshaft member 102 is rotated, the pressure of a lubricating oil filmformed the radial bearing gap between a cylinder face 8 a formed on theinner periphery of the bearing sleeve 8 and the radial hydrodynamicgroove regions 123 a, 123 b of the shaft portion 121 facing the cylinderface 8 a is increased by the hydrodynamic effect of the hydrodynamicgrooves 123 a 1, 123 b 1. Subsequently, a first radial bearing portionR11 and a second radial bearing portion R12 which rotatably support theshaft member 102 in the radial direction in a non-contact manner areformed by the pressure of these oil films. Moreover, the pressure of thelubricating oil films formed the thrust bearing gap between the lowerend face 8 b of the bearing sleeve 8 and the thrust hydrodynamic grooveregion 122 a of the upper side (the shaft portion side) of the flangeportion 122 facing the lower end face 8 b, and the thrust bearing gapbetween the upper end face 7 b 1 of the bottom 7 b and the thrusthydrodynamic groove region 122 b of the lower side (opposite to theshaft portion side) of the flange portion 122 facing the upper end face7 b 1 is increased by the hydrodynamic effect of the hydrodynamicgrooves 122 a 1, 122 b 1. Subsequently, a first thrust bearing portionT11 and a second thrust bearing portion T12 which rotatably support theshaft member 102 in the thrust direction in a non-contact manner areformed by the pressure of these oil films.

A method for producing of the shaft member 102 constituting the abovehydrodynamic bearing apparatus 101 will be described below.

The shaft member 102 is produced in mainly two steps: (C) forming stepand (D) grinding step. Among them, the (C) forming step comprises ashaft material forming process (C-1), a thrust hydrodynamic grooveregion forming process (C-2), a radial hydrodynamic groove regionforming process (C-3), and a shaft portion correcting process (C-4). The(D) grinding step comprises a width grinding process (D-1), a full-facegrinding process (D-2), and a finish grinding process (D-3).

(C) Forming Step

(C-1) Shaft Material Forming Process and (C-2) Thrust HydrodynamicGroove Region Forming Process

To begin with, a material of the shaft member 102 to be formed, i.e., ametal material such as stainless steel is compression-formed (forgingprocess) by using molds, for example, as shown in FIG. 17, in a coldstate, whereby the shaft material 110 integrally having the region 111corresponding to the shaft portion (hereinafter referred to simply as ashaft portion) and the region 112 corresponding to the flange portion(hereinafter referred to simply as a flange portion) is formed (shaftmaterial forming process (C-1)). The molds used in the forge forming ofthis shaft material 110 also serves as the molds for forming thrusthydrodynamic groove regions 112 a, 112 b on the flange portion 112 inthis embodiment. Accordingly, simultaneously with the forge forming ofthe shaft material 110, plastic processing is performed in the positionscorresponding to both end faces of the flange portion 112. For example,as shown in FIGS. 18 and 19, thrust hydrodynamic groove regions 112 a(the shaft portion side), 112 b (opposite to the shaft portion side)comprising a plurality of hydrodynamic groove 112 a 1, 112 b 1 anddemarcation portions 112 a 2, 112 b 2 demarcating these hydrodynamicgrooves 112 a 1, 112 b 1 are formed (thrust hydrodynamic groove regionformation process (C-2)).

A method of cold-forging employed in the above forming step may beextrusion, upsetting, heading or the like, or combinations of them. Inthe example shown in FIG. 17, the outer circumferential surface lila ofthe shaft portion 111 after the forging process has a different diametershape in which a tapered face 114 and a cylinder face 115, which isupwardly continuous with the tapered face 114 and has a diameter smallerthan other portions, are disposed therebetween, and the tapered face 114may be dispensed with and formed to have a uniform dimer throughout itslength. Note that described in this embodiment is the case where theforming of the shaft material 110 and the forming of the thrusthydrodynamic groove regions 112 a, 112 b are conducted simultaneously bythe forging process. However, both steps need not necessarily beperformed simultaneously, and the thrust hydrodynamic groove regions 112a, 112 b may be formed by plastic processing, for example, a forgingprocess, pressing process or the like after forming the shaft material110 by forging.

(C-3) Radial Hydrodynamic Groove Region Forming Process and (C-4) ShaftPortion Correcting Process

The shaft portion 111 of the shaft material 110 formed by forging in theprevious step is pressurized a pair of rolling dies (for example, rounddies, flat dies, etc.), for example, in the shape shown in FIGS. 6 or 7and the pair of rolling dies are reciprocated in the directions oppositeto each other so that a hydrodynamic groove transcription facepreviously formed on the holding face of either of the pair of rollingdies are transcribed (radial hydrodynamic groove region forming process(C-3)) on the outer circumferential surface 111 a of the shaft portion111. Since the above pair of rolling dies in this embodiment also servesas a correcting tool for correcting the shaft portion 111 of the shaftmaterial 110, a rolling process for correcting cylindricity is conducted(shaft portion correcting process (C-4)) on the outer circumferentialsurface 111 a of the shaft portion 111 simultaneously with transcriptionof the above hydrodynamic grooves.

As a result, for example, radial hydrodynamic groove regions 113 a, 113b having the shape shown in FIG. 17 are formed at two axially separatedpositions on the outer circumferential surface 111 a of the shaftportion 111, while out of the outer circumferential surface 111 a of theshaft portion, a face 113 including radial hydrodynamic groove regions113 a, 113 b (for example, the bottom faces of hydrodynamic grooves 113a 1, 113 b 1 and the outer circumferential surfaces of demarcationportions 113 a 2, 113 b 2 demarcating the hydrodynamic grooves 113 a 1,113 b 1) is corrected, and the cylindricity of the face 113 subjected tothe correcting process is improved to be within a desired range (forexample, 10 μm or lower). Simultaneously, the cylinder face 115 of theupper end of the shaft portion 111 is also subjected to a correctingprocess, and the cylindricity of the cylinder face 115 is improvedsimilarly.

As mentioned above, forming of the radial hydrodynamic groove regions113 a, 113 b and correction of the outer circumferential surface 111 aof the shaft portion can be both performed simultaneously by rolling.Additionally, for example, after a correcting process is performed onthe outer circumferential surface 111 a of the shaft portion 111, aprocedure to perform a rolling process of the radial hydrodynamic grooveregions 113 a, 113 b on the face subjected to the correcting process canbe also employed. In that case, various processing methods including arolling process, drawing, ironing, sizing by pressing split-cavity molds(clipping) and the like, can be employed in the correcting process ofthe cylindricity. Moreover, the correcting process is performedthroughout the length of the outer circumferential surface 111 a of theshaft portion 111, or can be conducted on a part of the outercircumferential surface 111 a as long as the part includes the radialhydrodynamic groove regions 113 a, 113 b.

As mentioned above, the forming of the shaft material 110 integrallycomprising the shaft portion 111 and flange portion 112 and the formingof the thrust hydrodynamic groove regions 112 a, 112 b on both end facesof the flange portion 112 are simultaneously performed both by forging,and in addition, the forming of the radial hydrodynamic groove regions113 a, 113 b and the correcting process of the outer circumferentialsurface 111 a of the shaft portion are performed simultaneously both byrolling, whereby such processing steps can be simplified and machiningtime can be greatly shortened. Moreover, compared to cutting or etching,etc., employing forging processes and rolling processes in which thecycle time per processed item is shorter can further shorten themachining time, enabling further cost reduction and improvement in massproductivity.

At the stage where the above forming step (C) has been completed, forexample, as shown in FIG. 20, the height h1 from the bottom face 112 b 3of the hydrodynamic groove 112 b 1 to the axial end face 112 b 4 of thedemarcation portion 112 b 2 in the thrust hydrodynamic groove region 112b is set to a suitable value considering the forming accuracy in theabove forging process and the grinding allowance in the width grinding(D-1) of the shaft material 110 described later. The height (not shown)from the bottom faces of the hydrodynamic grooves 113 a 1, 113 b 1 inthe radial hydrodynamic groove regions 113 a, 113 b to the outercircumferential surfaces of the demarcation portions 113 a 2, 113 b 2,and the height (not shown) from the bottom faces of the hydrodynamicgroove 112 a 1 in the thrust hydrodynamic groove region 112 a on theshaft portion 111 side to the axial end faces of the demarcation portion112 a 2 are set to suitable values considering the forming accuracy inthe above forging process, and the full-face grinding (D-2) of the shaftmaterial 110 described later and the grinding allowance in the finishgrinding (D-3).

(D) Grinding Step

(D-1) Width Grinding

The end face on the side opposite to the shaft portion on the side onwhich the end face 111 b of the shaft portion and the thrusthydrodynamic groove region 112 b of the flange portion 112, which willbe the two end faces of the shaft material 110 after being subjected tothe forming step are formed (refer to FIG. 19) is ground relative to thecorrected face 113 mentioned above. A grinding apparatus used in thisgrinding step comprises, as shown in FIGS. 8 and 9, a carrier 41retaining a plurality of the shaft materials 110 as workpieces; and apair of grind stones 42, 42 which grind the end face opposite to theshaft portion side comprising the end face 111 b of the shaft portion ofthe shaft material 110 retained by the carrier 41 and the thrusthydrodynamic groove region 112 b of the flange portion 112, as in thefirst embodiment. Note that other constitutions of the grindingapparatus 40 than this are based on the first embodiment, and theirexplanations are thus omitted.

As the carrier 41 rotates, the shaft material 110 is sequentially loadedinto the notch 43 from a fixed position. The loaded shaft material 110traverses the end faces of the rotating grind stones 42, 42 from theirouter diameter edge toward the inside diameter edge, while beingprevented from falling off from the notch 43 by binding of the belt 44.Accordingly, both end faces of the shaft material 110, namely the endface 111 b of the shaft portion and the end face of the flange portion112 on the side opposite to the shaft portion comprising the thrusthydrodynamic groove region 112 b are ground by the end faces of thegrind stones 42, 42 (refer to FIG. 9). Moreover, the width of the shaftmaterial 110 in the axial direction (the entire length including theflange portion 112) is finished to have a predetermined size.

In this grinding step, as mentioned above, the thrust hydrodynamicgroove region 112 b of the flange portion 112 is ground, for example, insuch a manner that the demarcation portion 112 b 2 is ground by apredetermined grinding allowance (h1-h2 in FIG. 21) from the height h1at the time of forging, as shown in FIG. 21. This renders the height ofthe demarcation portion 112 b 2 (the depth of the hydrodynamic groove112 b 1) to be the same as the predetermined value h2 (for example, 3μm-15 μm). Therefore, the thrust bearing gap between the componentfacing it (in this embodiment, the bottom 7 b of the housing 7) anditself can be controlled highly accurately at the interval of a severalmicrometers to several ten micrometers.

(D-2) Full-Face Grinding Process

Subsequently, relative to the ground two end faces of the shaft material110 (the end face 111 b of the shaft portion, the end face of the flangeportion 112 on the side opposite to the shaft portion comprising thethrust hydrodynamic groove region 112 b), the outer circumferentialsurface 110 a of the shaft material 110 and the end face of the flangeportion 112 on the shaft portion side comprising the thrust hydrodynamicgroove region 112 a are ground. A grinding apparatus used in thisgrinding step conduct plunge-grinding by the grind stone 53, with theback plate 54 and pressure plate 55 pressed against both end faces ofthe shaft material 110, as in the first embodiment shown in FIG. 10. Thecorrected face 13 of the shaft material 110 is rotatably supported by ashoe 52. Note that other constitutions of the grinding apparatus 50 thanthis is based on the first embodiment and their explanations are thusomitted.

Grinding in the grinding apparatus 50 of the above constitution isperformed in the following procedure. To begin with, while the shaftmaterial 110 and the grind stone 53 are in rotation, the grind stone 53is fed obliquely (the direction of arrow 1 in FIG. 10), the planegrinding portion 56 b of the grind stone 53 is pressed against the endface of the flange portion 112 on the shaft portion side of the shaftmaterial 110, the end face of the flange portion 112 on the shaftportion side (on the thrust hydrodynamic groove region 112 a side)comprising the thrust hydrodynamic groove region 112 a is ground.Accordingly, the end face of the flange portion 122 of the shaft member102 on the shaft portion side is formed, and grinding of the thrusthydrodynamic groove region 112 a is completed, and the thrusthydrodynamic groove region 122 a of the shaft member 102 is formed.Subsequently, the grind stone 53 is fed in the direction perpendicularlyintersecting the rotation axis of the shaft material 110 (the directionof arrow 2 in FIG. 10), the cylinder grinding portion 56 a of the grindstone 53 is pressed against the outer circumferential surface 111 a ofthe shaft portion 111 of the shaft material 110 and the outercircumferential surface 112 c of the flange portion 112 to grind thefaces 111 a, 112 c. Accordingly, out of the outer circumferentialsurface of the shaft portion 121 of the shaft member 102, the radialhydrodynamic groove region 123 a, 123 b and the region corresponding tothe cylinder face 125 are ground, while the tapered face 124, the outercircumferential surface 122 c of the flange portion 122, and the recessportions 126-128 are further formed.

In this grinding step (full-face grinding process), the demarcationportion 112 a 2 of the thrust hydrodynamic groove region 112 a formed onthe end face of the flange portion 112 on the shaft portion side isground, for example, by a predetermined grinding allowance from theheight at the time of forging, similarly to the case of the thrusthydrodynamic groove region 112 b, although not shown in the Figs. Thisrenders the height of the demarcation portion 112 a 2 (the depth of thehydrodynamic groove 112 a 1) to have a predetermined value, whereby thethrust bearing gap between the component facing it (the lower end face 8b of the bearing sleeve 8 in this embodiment) and itself is highlyaccurately controlled. In this embodiment, since the accuracy setting ofthe perpendicularity of both end faces of the shaft material 110 (theend face 111 b of the shaft portion, the end face of the flange portion112 on the side opposite to the shaft portion) has been performedpreviously in the width grinding process, grinding of the thrusthydrodynamic groove region 112 a can be conducted more precisely.

(D-3) Finish Grinding Process

(D-2) Among the faces which have been ground in full-face grindingprocess, the radial hydrodynamic groove regions 123 a, 123 b of theshaft member 102 and the region corresponding to the cylinder face 125are subjected to the final finish grinding. As in the first embodiment,a grinding apparatus used in this grinding is a cylinder grinder shownin FIG. 11. It performs plunge grinding by the grind stone 63 whilerotating the shaft material 110 held between the back plate 64 and thepressure plate 65. Note that other constitutions of the grindingapparatus 60 are based on the first embodiment, and their explanationsare thus omitted.

In the grinding apparatus 60 having the above constitution, the rotatinggrind stone 63 is provided with the feed in the radial direction so thatthe radial hydrodynamic groove regions 123 a, 123 b and the regions 113a, 113 b and 115 corresponding to the cylinder face 125 are ground, andthese regions are finished to have the final surface accuracy. In thisgrinding step, similarly to the case of the thrust hydrodynamic grooveregions 112 a, 112 b, the demarcation portions 113 a 2, 113 b 2 of theradial hydrodynamic groove regions 113 a, 113 b is ground, for example,by a predetermined grinding allowance from the height at the time ofrolling, although not shown in the Figs. This renders the heights of thedemarcation portions 113 a 2, 113 b 2 (the depth of hydrodynamic grooves113 a 1, 113 b 1) to have a predetermined value, enabling to highlyaccurately control the radial bearing gap between the component facingit (in this embodiment, the cylinder face 8 a of the bearing sleeve 8)and itself.

After being subjected to the above (C) forming step and (D) grindingstep, the shaft member 102 shown in FIG. 13 is completed by performing,if necessary, heat treatment and cleaning process.

The shaft member 102 produced by the above production method has theradial hydrodynamic groove regions 123 a, 123 b formed at two separateupper and lower portions on the outer periphery of the shaft portion 121by a rolling process, and has such a structure that the outercircumferential surfaces of the demarcation portions 123 a 2, 123 b 2 ofthe radial hydrodynamic groove regions 123 a, 123 b are the grindingsurfaces. It also has the thrust hydrodynamic groove regions 122 a, 122b formed by a forging process on both end faces of the flange portion122, and has such a structure that the axial end faces of the thrusthydrodynamic groove regions 122 a, 122 b are the grinding surfaces. Thegrinding surfaces of the demarcation portions 123 a 2, 123 b 2 in theradial hydrodynamic groove regions 123 a, 123 b are formed in the (D-2)full-face grinding process and (D-3) finish grinding process. Moreover,the grinding surface of the demarcation portion 122 a 2 in the thrusthydrodynamic groove region 122 a is formed in the (D-2) full-facegrinding process, and the grinding surface is formed in the (D-1) widthgrinding process of the demarcation portion 122 b 2 in the thrusthydrodynamic groove region 122 b.

As mentioned above, the radial hydrodynamic groove regions 113 a, 113 bof the shaft material 110 are formed by a rolling process, and among theradial hydrodynamic groove regions 113 a, 113 b, the outer diameterportions of the demarcation portions 113 a 2, 113 b 2 are ground,whereby the hydrodynamic grooves region 123 a, 123 b can be formed atreduced costs, while the dimensional accuracy of their outer diametersand surface roughness can be highly accurately finished. As for thethrust hydrodynamic groove regions 122 a, 122 b, low-cost forming andhigh-accuracy finish can be achieved at the same time for the samereason. This allows the radial bearing gap and thrust bearing gap in thehydrodynamic bearing apparatus 101 to be controlled highly accurately,enabling to produce stable bearing performance.

According to the above production method, it is also possible to highlyaccurately finish the cylindricity of the radial hydrodynamic grooveregions 123 a, 123 b formed on the outer periphery of the shaft portion121. Accordingly, for example pressure, variation of the radial bearinggap formed between the cylinder face 8 a of the inner periphery of thebearing sleeve 8 in the bearing apparatus 101 and the hydrodynamicgroove regions in the circumferential direction or axial direction issuppressed to fall within a predetermined range, and bearing performancecan be prevented from being adversely affected by the variation of theabove radial bearing gap. Moreover, the grinding allowance of thedemarcation portion in grinding (h1-h2 in FIG. 21) varies depending onthe forming accuracy in forging or rolling. As shown in this embodiment,the cylindricity of the shaft portion 121 is corrected so that inparticular the forming accuracy of the demarcation portions 123 a 2, 123b 2 in the radial hydrodynamic groove region 123 a, 123 b can beimproved and the grinding allowance in grinding can be reduced. Thisenables to further shorten machining time and reduce processing costs.Alternatively, the forming accuracy of the hydrodynamic groove region inforging or rolling is preliminarily increased, whereby the grindingallowance in grinding can be reduced.

As mentioned above, if the radial hydrodynamic groove regions 123 a, 123b are formed on the outer periphery of the shaft member 102,hydrodynamic grooves need not be processed on the inner periphery of thebearing sleeve 8. The inner periphery of the bearing sleeve 8 can serveas the cylinder face 8 a, reducing such related costs. Moreover, ifhydrodynamic grooves need not be processed on the inner periphery of thebearing sleeve 8, it is unnecessary to form the bearing sleeve 8 and thehousing 7 as separate components. Therefore, these components can beunified (with a resin or the like), although not shown in the Figs. Thiscan reduce the number of parts and related production costs.

In the second embodiment described above, the case where the radialhydrodynamic groove regions 113 a, 113 b are formed by a rolling processis described, but alternatively, for example, the forging of the shaftmaterial 110 and the thrust hydrodynamic groove regions 112 a, 112 b canbe conducted simultaneously with the forming of the radial hydrodynamicgroove regions 113 a, 113 b by forging. In this case, the shape of thehydrodynamic grooves by forging is not particularly limited, and may be,for example, a herringbone shape, a spiral shape, or other varioushydrodynamic groove shapes.

In the second embodiment, described was the case where the thrusthydrodynamic groove regions 122 a, 122 b are formed on both end faces ofthe flange portion 122. However, it is not particularly limited to thisform, and, for example, the thrust hydrodynamic groove regions may beformed on the side of the lower end face 8 b of the bearing sleeve 8 andthe upper end face 7 b 1 of the bottom 7 b facing the two end faces ofthe flange portion 122, respectively.

In the embodiments described above (the first and second embodiments),for example, bearings using hydrodynamic pressure producing partscomprising hydrodynamic grooves arranged in a herringbone shape and aspiral shape are shown as examples of the hydrodynamic bearing whichconstitutes the radial bearing portions R1, R2, R11, R12 and the thrustbearing portions T1, T2, T11, T12. However, the constitution of thehydrodynamic pressure producing parts is not limited to these. As theradial bearing portions R1, R2, R11, R12, for example, multirobebearing, step bearing, taper bearing, taper flat bearing or the like maybe used. As the thrust bearing portions T1, T2, T11, T12, step pocketbearing, taper pocket bearing, taper flat bearing and the like may beused.

In the embodiments described above, a lubricating oil is mentioned as anexample of a fluid which fills the inside of the hydrodynamic bearingapparatus 1, 101 and produces hydrodynamic effect in the radial bearinggap between the bearing sleeve 8 and the shaft member 2, 102 and thethrust bearing gap between the bearing sleeve 8 and housing 7 and theshaft member 2, 102. However, it is not particularly limited to thisfluid. A fluid which can produce hydrodynamic effect in the bearing gapshaving hydrodynamic groove regions, for example, a gas such as air and alubricant having fluidity such as a magnetic fluid may be used.

The hydrodynamic bearing apparatus according to the present invention issuitable for spindle motors of information appliances, for example,magnetic disk apparatuses such as HDD, optical disk apparatuses such asCD-ROM, CD-R/RW, DVD-ROM/RAM, magneto-optic disk apparatuses such as MDand MO, etc., polygon scanner motors of laser beam printers (LBP), andother small motors.

1. A shaft member for hydrodynamic bearing apparatuses comprising ashaft portion and a flange portion each formed by forging, a radialbearing face which faces a radial bearing gap and is formed on the outerperiphery of the shaft portion, and said radial bearing face having acylindricity of 3 μm or lower.
 2. A shaft member for hydrodynamicbearing apparatuses according to claim 1, wherein the perpendicularityof both end faces of the flange portion and the perpendicularity of anend face of the shaft portion relative to said radial bearing face areeach 5 μm or lower.
 3. A shaft member for hydrodynamic bearingapparatuses according to claim 1, wherein said shaft portion and flangeportion are integrally formed by forging.
 4. A shaft member forhydrodynamic bearing apparatuses according to claim 1, wherein both endfaces of said shaft member are grinding surfaces.
 5. A shaft member forhydrodynamic bearing apparatuses according to claim 1, wherein aslanting recess portion is formed at the corner between said shaftportion and flange portion.
 6. A hydrodynamic bearing apparatuscomprising a shaft member for hydrodynamic bearing apparatuses accordingto claim 1; a bearing sleeve into which said shaft member is inserted atits inner surface; a radial bearing portion which produces pressure bythe hydrodynamic effect of a fluid which occurs in a radial bearing gapbetween the outer periphery of the shaft portion and the inner surfaceof the bearing sleeve to support the shaft portion in the radialdirection in a non-contact manner; a first thrust bearing portion whichproduces pressure by the hydrodynamic effect of a fluid which occurs ina thrust bearing gap on one end side of the flange portion to supportthe flange portion in the thrust direction in a non-contact manner; anda second thrust bearing portion which produces pressure by thehydrodynamic effect of a fluid occurring in the thrust bearing gap onthe other end side of the flange portion to support the flange portionin the thrust direction in a non-contact manner.
 7. A hydrodynamicbearing apparatus according to claim 6, wherein hydrodynamic grooves forproducing the hydrodynamic effect of the fluid are formed asymmetricallyin the axial direction on one of the outer circumferential surface ofthe shaft portion facing the radial bearing gap and the inner surface ofthe bearing sleeve facing this outer circumferential surface.
 8. A motorcomprising the hydrodynamic bearing apparatus according to claim 6, arotor magnet and a stator coil.
 9. A method for producing a shaft memberfor hydrodynamic bearing apparatuses, the method comprising the step offorming the shaft material which integrally has the shaft portion andthe flange portion by a forging process; and the step of correcting thecylindricity of a part or the entire outer circumferential surface ofthe shaft portion.
 10. A method for producing a shaft member forhydrodynamic bearing apparatuses according to claim 9, wherein saidcorrecting step is performed by rolling.
 11. A method for producing ashaft member for hydrodynamic bearing apparatuses according to claim 9,wherein a first grinding process is performed on both end faces of theshaft material relative to said corrected face, and a second grindingprocess is performed on at least the outer circumferential surface ofthe shaft material relative to said both end faces.
 12. A method forproducing a shaft member for hydrodynamic bearing apparatuses accordingto claim 11, wherein the first grinding process is performed on theother hand end face of the flange portion and on the end face of theshaft portion.
 13. A method for producing a shaft member forhydrodynamic bearing apparatuses according to claim 11, wherein saidsecond grinding process is performed on at least a portion which servesas a radial bearing face facing a radial bearing gap on the outerperiphery of the shaft portion of the shaft material.
 14. A method forproducing a shaft member for hydrodynamic bearing apparatuses accordingto claim 13, wherein the other end face of the flange portion is groundfurther in the second grinding process.
 15. A metallic shaft member forhydrodynamic bearing apparatuses which integrally comprises a shaftportion and a flange portion, and a radial hydrodynamic groove regionwhich comprises a plurality of hydrodynamic grooves and demarcationportions demarcating each hydrodynamic groove being formed on the outerperiphery of said shaft portion by plastic processing, and said outercircumferential surfaces of the demarcation portions in the radialhydrodynamic groove region being grinding surfaces.
 16. A shaft memberfor hydrodynamic bearing apparatuses according to claim 15, wherein athrust hydrodynamic groove region comprising a plurality of thehydrodynamic grooves and demarcation portions demarcating eachhydrodynamic groove is formed by plastic processing on both end faces ofsaid flange portion, and the end face in the axial direction of thedemarcation portion in said thrust hydrodynamic groove region is agrinding surface.
 17. A shaft member for hydrodynamic bearingapparatuses according to claim 15, wherein said radial hydrodynamicgroove region is formed by a rolling process or a forging process.
 18. Ashaft member for hydrodynamic bearing apparatuses according to claim 16,wherein said thrust hydrodynamic groove region is formed by a forgingprocess.
 19. A shaft member for hydrodynamic bearing apparatusesaccording to claim 15, wherein said shaft portion and said flangeportion are integrally formed by forging.
 20. A hydrodynamic bearingapparatus comprising a shaft member for hydrodynamic bearing apparatusesaccording to claim 15; and a sleeve member into which said shaft memberis inserted at its inner surface to form a radial bearing gap betweenitself and said shaft member, wherein said shaft member and said sleevemember being retained in a non-contact manner by said hydrodynamiceffect of the fluid occurring in the radial bearing gap.
 21. Ahydrodynamic bearing apparatus according to claim 20, wherein the sleevemember is formed of an oil-containing sintered metal.
 22. A hydrodynamicbearing apparatus according to claim 20, wherein hydrodynamic groovesfor producing the hydrodynamic effect of the fluid are formedasymmetrically in the axial direction on the outer circumferentialsurface of the shaft portion facing the radial bearing gap.
 23. A motorcomprising a hydrodynamic bearing apparatus according to claim 20, arotor magnet and a stator coil.
 24. A method for producing a shaftmember for hydrodynamic bearing apparatuses comprising a shaft portionand a flange portion integrally, and a radial hydrodynamic groove regionwhich comprises a plurality of hydrodynamic grooves and demarcationportions demarcating each hydrodynamic groove being formed on the outerperiphery of said shaft portion, the method comprising forming saidradial hydrodynamic groove region by plastic processing on the outerperiphery of the shaft portion of the shaft material, and then grindinga portion including the outer diameter portion of the demarcationportion in said radial hydrodynamic groove region.
 25. A method forproducing a shaft member for hydrodynamic bearing apparatuses accordingto claim 24, the method comprising forming said shaft material and saidradial hydrodynamic groove region both by forging, and simultaneouslyperforming forging of both.
 26. A method for producing a shaft memberfor hydrodynamic bearing apparatuses according to claim 24, whereinforming said radial hydrodynamic groove region and correcting saidcylindricity of a portion including the radial hydrodynamic grooveregion of the shaft portion are both performed by rolling, and therolling process of both are simultaneously performed.
 27. A method forproducing a shaft member for hydrodynamic bearing apparatuses accordingto claim 24, wherein forming said shaft material and forming the thrusthydrodynamic groove region comprising the hydrodynamic grooves anddemarcation portions demarcating each hydrodynamic groove on both endfaces of the flange portion are both performed by forging, and theforging process of both is performed simultaneously.